1. Field of the Invention
The present invention is directed to a mobile X-ray apparatus with an X-ray system having an X-ray source and an X-ray detector, of the type wherein the X-ray system is arranged at a carrier device so as to be adjustable relative to a subject for registering a series of 2D projections of the subject for the reconstruction of a 3D image of the subject. The invention is also directed to a method for determining projection geometries for such an X-ray apparatus.
2. Description of the Prior Art
X-ray apparatuses of the above type, which are usually a mobile C-arm X-ray apparatus, are used, for example, in medicine to reconstruct 3D images of a body part of a patient from a series of 2D projections of the body part registered from different projection angles. The reconstruction of 3D images from the 2D projections registered with the X-ray system, however, assumes a knowledge of the projection geometries, i.e. knowledge of the positions of the X-ray source and of the X-ray detector, as well as knowledge of the projection angle at each of the individual 2D projections. Since C-arm X-ray apparatuses exhibit mechanical instabilities, particularly relating to the adjustment motion of the C-arm, which are expressed in twisting of the C-arm given adjustment motions thereof due to the weight of the X-ray source and of the X-ray receiver, special measures must be provided in order to determine the projection geometries during the 2D projections, and thus to allow reconstruction of 3D images of the subject.
German OS 197 46 093 discloses a C-arm X-ray apparatus of this type. For registering a series of 2D projections, the C-arm of the X-ray apparatus provided with the X-ray system is preferably adjusted isocentrically along its circumference, known as an orbital motion, under motor drive in an angular range of approximately 200xc2x0 around the body part of the patient to be examined. For determining the projection geometries, the X-ray apparatus has acoustic or electromagnetic transmitter devices arranged at the X-ray source and at the X-ray receiver, and receiver devices arranged at components of the X-ray apparatus that are stationary relative to the X-ray system. The projection geometries of the individual 2D projections are determined by transit time or phase measurements of the acoustic or electromagnetic waves between the transmitter and receiver devices during the course of registering 2D projections, and 3D images of the examined subject are thus generated from the series of 2D projections.
It has proven disadvantageous in the known X-ray apparatus that the realization of the motorized adjustment of the C-arm along its circumference is technically complicated and thus expensive. Moreover, the determination of the projection geometries during the registration of a series of 2D projections, which is also referred to as online determination of the projection geometries, requires a high use of computing power in order to obtain 3D images of the subject as desired in an optimally short time after the registration of the series of 2D projections. A real-time reconstruction of 3D images, accordingly, is only possible with utilization of an expensive computer exhibiting high computing performance.
An object of the present invention is to provide an X-ray apparatus of the type initially described wherein the technical outlay for producing 3D images of a subject is reduced. A further object of the invention is to provide a method with which the projection geometries required for the reconstruction of 3D images can be made available for the inventive X-ray apparatus in a simplified way.
According to the invention, the first object is achieved in a movable (mobile) X-ray apparatus having an X-ray system with an X-ray source and an X-ray detector that is arranged at a carrier, the carrier being pivotable under motor drive around an axis proceeding substantially horizontally through the carrier device for registering a series of 2D projections of a subject, and means for producing a 3D image dataset from the registered 2D projections. Differing from known X-ray apparatuses, the carrier device of the inventive X-ray apparatus is pivotable under motor drive around an axis proceeding substantially horizontally through the carrier. The form of pivot around the horizontally proceeding axis, which corresponds to the angulation axis of the C-arm in an embodiment of the carrier of the inventive X-ray apparatus, can be significantly more simply realized than the technically complex, motorized orbital rotation of the C-arm, which is understood by those skilled in the art as adjustment of the C-arm along its circumference. As a result of the inventive fashioning of the X-ray apparatus, accordingly, the technical outlay for producing a 3D image dataset of a subject from which 3D images of the subject can be acquired is significantly reduced.
Another advantage of the invention is that a simple C-arm X-ray apparatus having a non-isocentrically adjustable C-arm can be implemented without complications by a C-arm that is pivotable around its angulation axis under motor drive, so that 3D image datasets for the reconstruction of 3D images of a subject can be acquired with such simple X-ray apparatus. For a non-isocentrically C-arm the position of the intersection between the central ray of an X-ray beam emitted by the X-ray source and the angulation axis does not change appreciably given pivoting of the C-arm around the angulation axis, as is the case given an orbital rotation of an isocentrically adjustable C-arm around the isocenter of the C-arm. As a result, the generation of a 3D image dataset from a series of 2D projections is simplified in both cases.
In a preferred embodiment of the invention the motorized pivot of the carrier device is effected by a digitally controlled drive. The drive, which is preferably controlled by software and which includes a stepping motor according to one embodiment of the invention, enables a precise pivoting of the carrier device around the horizontally proceeding axis, and various pivoted positions of the carrier device can be repeatedly set with high precision. Individual pivot positions in the adjustment motion of the C-arm can be repeatedly approached with a precision of up to 500 xcexcxc2x0 with the stepping motor.
The object directed to the method is achieved in a method for determining the projection geometries for a mobile X-ray apparatus, wherein the carrier device is pivotable, relative to a holder of the X-ray apparatus, around an axis proceeding substantially horizontally through the holder and the carrier device, the method including the following method steps:
a) setting first exposure parameters covering the initial position of the carrier device relative to the holder;
b) arranging a phantom provided for determining the projection geometries relative to the X-ray system so that it can be penetrated by an X-ray beam proceeding from the X-ray source to the X-ray detector;
c) registering a series of 2D projections of the phantom during the motorized pivot of the carrier device relative to the holder around the horizontally proceeding axis;
d) interpreting the registered 2D projections of the phantom for determining the projection geometries for each of the 2D projections;
e) storing the determined projection geometries for the first exposure parameters; and
f) repeating steps a) through e) as needed for modified exposure parameters.
When the carrier device of the X-ray apparatus is fashioned as C-arm that is mounted so as to be adjustable along its circumference in a support connected to the holder, in one version of the method for determining the projection geometries that the determination of the projection geometries be undertaken dependent on the position of the carrier device relative to the support.
In the inventive method, differing from known X-ray apparatus operating methods, the projection geometries are determined and stored in one or more calibration events preceding the registration of series of 2D projections of the subject instead of during the registration of a series of 2D projections of the subject, in order to make the projection geometries available for later measurements of series of 2D projections of subjects for generating 3D image datasets. The inventive method thus is based on the recognition that the twisting of the carrier device which occurs during the adjustment of the carrier device around the horizontally proceeding axis of the X-ray apparatus, as a result of the weight of the X-ray source and of the X-ray detector, can be considered as a mechanical constant, this twisting leading to deviations from the ideal adjustment motion of the X-ray system around the subject. Since the carrier device always exhibits approximately the same twisting given swivel motions under the same initial conditions relating to the adjustment motion of the carrier device, i.e. given the same exposure parameters, which also include the initial position of the carrier device, the pivot velocity of the carrier device, the approach and braking curve of the carrier device as well as the pivot angle, this assumption has proven justified. The pivot motion of the carrier device in the X-ray system around the horizontally proceeding axis can, accordingly, be considered reproducible. The projections geometries of the inventive X-ray apparatus therefore can be determined in one or more calibration events before the actual subject measurements, which is also referred to as offline determination of the projection geometries.